A low refractive index film, which is formed on the surface of a transparent substrate such as glass or plastic, is used as an antireflection film for preventing reflection of incident light in display panels such as a cathode ray tube, liquid crystals, and an organic EL; glass for solar cells, optical lens, and a showcase; and the like. For example, the display surface of the display panel is covered with the antireflection film for improving visibility. Alternatively, in the field of solar cells, in order to prevent reflection of incident sunlight and increase the absorptance of light, a low refractive index film is formed as the antireflection film on the surface and the like of a glass substrate.
As such an antireflection film, a monolayer film, which is formed by a gas-phase method such as a vacuum deposition method or a sputtering method and formed of MgF2, cryolite, or the like, has been commercialized in the related art. Moreover, a multilayer film or the like, which is formed by alternately layering a low refractive index coat such as SiO2 and a high refractive index coat such as TiO2 or ZrO2 on a substrate, is also known to produce a strong antireflection effect. However, the gas-phase method such as a vacuum deposition method or a sputtering method has a problem in terms of the production costs and the like since the apparatus and the like used for the method are expensive. Further, the method of forming a multilayer film by alternately layering a low refractive index coat and a high refractive index coat is not so practical since the production process thereof is complicated and the method takes a lot of time and trouble.
Accordingly, recently, in view of the production costs and the like, a coating method such as a sol-gel method has drawn attention. In the sol-gel method, generally, a sol-gel solution is prepared, a transparent substrate such as glass is coated with the solution, and then the resultant is subjected to drying, firing, and the like to form a film. However, the film formed by the sol-gel method has many problems in that the refractive index thereof is not as low as that of the film formed by the gas-phase method such as a vacuum deposition method, and the film exhibits poor adhesiveness with respect to a substrate and cracks.
As the low refractive index film formed using the sol-gel method, there is a disclosure regarding low refractive index antireflection film which is formed of (a) silica sol in which silica particles having a predetermined particle size have dispersed and (b) at least one kind of component selected from a group consisting of a hydrolysate of alkoxysilane, a hydrolysate of metal alkoxide, and a metal salt, and formed by coating a substrate with a coating solution containing an organic solvent that contains the above components in an intended proportion and then curing the resultant (for example, see Patent Document 1). According to the disclosure, in this film, particularly by using the silica particles in a specific proportion, fine concavities and convexities are formed on the coat surface, and consequentially, while the refractive index is reduced, an excellent antireflection effect is obtained.